1. Field of the Invention
The present invention relates to the electronic control of the throttle valve of an internal combustion engine for controlling the opening of throttle valve with an actuator to meet the demand opening which is determined in accordance with certain information in addition to the depth of the accelerator pedal operated by the driver. More particularly, the invention relates to a throttle valve controller capable of actuating the throttle valve properly irrespective of the deviation of the actual opening from the demand opening.
2. Description of the Prior Art
The electronic throttle valve control, which is beneficial for the achievement of traction control and other engine control, is gaining the prevalence remarkably in recent years. A prior art example of electronic throttle valve control is described in Japanese patent Laid-Open No. Hei 7-293284. The throttle valve controller described in this patent publication fundamentally bases its control process on the known PID control scheme. Specifically, a table of control factors is arranged to provide the larger control factor when the deviation is smaller so that the throttle valve is driven only by the term of proportion (P term out of PID) even when the deviation is small.
Based on this variable control factor which is dependent on the deviation, it is possible to drive the throttle valve in response to the P term without waiting for the rise of the term of integration (I term out of PID) even against a small deviation, thereby quickly reducing the deviation, i.e., bringing the actual opening close to the demand opening.
The principle of the above-mentioned operation will be explained with reference to the graphs of FIG. 5. Graph (a) shows the transition of throttle valve operation in which the actual opening is coincident with the demand opening a.sub.2, i.e., zero deviation, until time point t.sub.0, at which the demand opening is stepped up slightly by 1.degree. to a.sub.3 as shown by the curve k. In case the control system adopts a fixed control factor, it uses the same control signal as for the larger deviation value, resulting in a P-term control signal rising merely to b.sub.1 as shown by the curve m on graph (b). This control signal is smaller than the minimum control signal b.sub.t for the actuator to produce a torque by which the throttle valve is moved to any extent. On this account, the actual opening stays at a.sub.2 until time point t.sub.1 when the I term rises significantly, and then the actual opening increases gradually and reaches the demand opening a.sub.3 at time point t.sub.2 as shown by the curve m on the graph (a). That is, this control system is inferior in output response when the deviation is small.
In contrast, in case the control signal adopts a variable control factor so that the larger control factor is produced against the smaller deviation, the P-term control signal rises to a large value b.sub.2 beyond the threshold level b.sub.t as shown by the curve n on the graph (b). Accordingly, the actual opening begins to vary immediately at time point t.sub.0 and reaches the demand opening a.sub.3 at time point t.sub.1 as shown by the curve n on the graph (a). That is, this control system is superior in output response even when the deviation is small.
However, the foregoing prior art throttle valve control cannot achieve stable transitional control inclusive of the output response in a wide range from a small deviation to a large deviation, i.e., another problem emerges when the deviation is large.
This affair will be explained with reference to the graphs of FIG. 6. In FIG. 6, graph (a) shows the transition of throttle valve operation in which the actual opening is coincident with the demand opening a.sub.1, i.e., the deviation is zero, until time point t.sub.0, at which the demand opening is stepped up greatly by 4.degree. to a.sub.3 as shown by the curve h. A resulting large deviation causes the P-term control signal to rise immediately to b.sub.3 beyond the threshold level b.sub.t as shown on graph (b), and the throttle valve begins to move to the demand opening a.sub.3. In case the control system adopts a fixed control factor, the control signal decreases gradually as the deviation value falls, and the actual opening reaches a.sub.3 stably as shown by the curve m on the graph (a). However, if the control factor is varied to be larger for the smaller deviation, as mentioned above, the control signal does not decrease as fast as the fall of the deviation value, causing the actual opening to exceed the demand opening a.sub.3 (overshoot) due to the inertia of the throttle valve and actuator as shown by the curve n on the graph (a). The overshooting triggers the oscillation of throttle valve opening, resulting in the oscillation of engine speed and the unstable engine operation.
This means that the prior art throttle valve control achieves high output response to the smaller deviation in exchange for low control stability against the larger deviation, and achieves high control stability against the larger deviation in exchange for lower output response to the smaller deviation. Although the conflicting control qualities associated with the deviation has been explained in connection with the P term of PID control, the same problem exists with the I term. Specifically, when the larger control factor is used against the smaller deviation so that the I term effectuate quickly with the intention of enhancing the output response to the smaller deviation, the I term works to excess against the diminishing deviation, creating the throttle valve movement to overshoot.